B cells are selected in the cell sorter according to the density of SIg and other glyco-proteins. Subsets with high and low density of surface molecules are stimulated with antigen and mitogen. The response, measured by alpha-aminoisobutyric acid uptake, thymidine uptake, glucose and amino acid transport, and Ca 2 ion efflux, is compared between cells of young and aged rats. Similarly, the molecules that aggregate at the point of contact between B and T lymphocytes and macrophage, during antibody formation, are identified to learn which ones are obligatory for such interactions and which are needed to signal antibody formation. Diffusion coefficients of SIg, RTl and Thyl are measured on lymphocyte membranes of aged and young rats. SIg is restricted in mobility by the cytoskeleton; RTl is partially restricted; Thyl has a mobility equal to small lipid molecules and is "free" in the cell membrane. The effect of aging on these mobilities will be examined.